1. Field of the Invention
The present invention relates to an electric control apparatus for a shock absorber disposed between unsprung mass and sprung mass of a suspension mechanism in an automotive vehicle and provided with an electrically operated control valve to be electrically controlled for changing damping characteristics of vertical displacement of the sprung mass relative to the unsprung mass.
2. Discussion of the Prior Art
Disclosed In Japanese Patent Laid-open Publication No. 3-276807 is a conventional electric control apparatus of this kind, wherein absolute vertical displacement velocity of the sprung mass and relative vertical displacement velocity of the sprung mass to the unsprung mass are detected to control the damping coefficient of the shock absorber on a basis of the skyhook theory proposed by D. Karnopp. In this control apparatus, an opening degree of the control valve is decreased in accordance with an increase of a difference between the absolute vertical displacement velocity and the relative vertical displacement velocity to increase the damping coefficient of the shock absorber and is increased in accordance with a decrease of the difference between the absolute vertical displacement velocity and the relative vertical displacement velocity to decrease the damping coefficient of the shock absorber.
In such control of the damping coefficient, however, the difference between the absolute vertical displacement velocity and the relative vertical displacement velocity changes from a positive value to a negative value or vice-versa if either the absolute vertical displacement velocity or the relative vertical displacement velocity changes in its sign from positive to negative or vice-versa. In such an instance, the opening degree of the control valve is greatly changed to cause sudden change of the damping coefficient of the shock absorber. In other words, when the relative vertical displacement velocity is maintained to be approximately zero, the damping force of the shock absorber is changed in a small range. In this instance, the passenger is not applied with any large shock. If the relative vertical displacement velocity is not maintained to be approximately zero, the damping force of the shock absorber changes in a large range. In this instance, the passenger is applied with a large shock.